For heavy-duty diesel engines where achieving maximum fuel pressure is desired, any trapped air within large fuel housings can pose a great engineering challenge. Air can become trapped within the fuel housing in a number of ways, including as a result of a new filter element being installed in the housing and air entrained in fuel entering the fuel filter housing.
Manual external or internal air-bleed valves have been developed to purge air outside of the filter housing. Various automatic air bleeding fuel filter designs are also known. The use of external air-bleed valves can present additional possibilities of functional failure where fuel-leaks could occur. Moreover, to manually bleed air out, some amount of fuel can spill out of the housing posing safety risks to the operator. The same safety risks apply to draining the fuel manually out of the housing during filter replacement.
In addition, fuel supply to a high pressure fuel pump typically utilizes part of the returned fuel from an in-built reservoir of the fuel filter housing which could contain air that's already vented out from the fuel filter housing. Some of this air can make it back into the Stage-1 filter through the standpipe which is on the upstream or dirty side of the filter media. If the media air-vent is located on the downstream or “clean-side” of the filter media, then the air-vent functionality will be reduced or eliminated altogether and air from the fuel supply could cause engine performance issues if it does not get vented out.